For a number of years, the treatment of household and industrial wastes has been the subject of special attention on the part of research and industrial bodies, because of the ever increasing health and environmental requirements. Attempts have thus been made to improve, inter alia, the processes of treatment of sewage, including that of the residual sludge which they produce.
Insofar as the problem of the elimination of residual sludge is concerned, a conventional process consists in thickening them and then stabilizing them either by aerobic digestion or by anaerobic mineralization, so as to oxidize the organic matter which they contain. In subsequent stages they are either dehydrated or incinerated or dumped or reemployed, for example in agriculture.
Examples of residual sludge which may be mentioned are sludges from purification stations and industrial sludges, such as the sludge originating from surface treatment plants, especially from electroplating plants.
Until now, the stage of drying of the sludge produced by purification stations, which treat household sewage mixed with rainwater and, possibly, with industrial water, has been carried out chiefly with the aid of lime, which makes it possible to reach a dryness value of approximately 22 to 25% after passing through a settler and a filter, such as a filter press or a belt filter. However, this known process is found to be unsuitable for the new discharge standards in force, which require a dryness higher than 35% and an absence of leaching of the contaminant, in particular of heavy metals. In addition, this technique does not make it possible to combat the odours of this sludge, which can be a considerable nuisance for the surrounding populations. Similar problems arise with regard to the sludge originating from physicochemical treatments in surface treatment, for example electroplating, workshops. This sludge has, in fact, a dryness of the order of 5% on leaving the settler and subsequently not exceeding 20 to 22% after passing through filter presses.
There is therefore a need for a process which makes it possible to overcome the above-mentioned disadvantages of the prior art.
A solution has been proposed in European patent application No. 0 337 047 (reputed to be withdrawn) for detoxifying liquid and solid media by removing the heavy metals which they contain. This solution consists in employing an active adsorbent material produced by an oxidation of anamorphosed aluminosilicate particles originating from coal residues. The oxidation may be performed, for example, by roasting in air in a fluidized-bed furnace at a temperature which is described as being between 350 and 800.degree. C. The active adsorbent material is prepared by granulation of the fly ash thus obtained, which initially has a particle size such that 34 to 45% of it has a diameter smaller than 100 .mu.m. The granules are subsequently employed as a filter bed in a percolation column.
In practice, however, the process which is the subject matter of this prior patent application has been implemented by employing ash produced by combustion, at a temperature of the order of 1100.degree.-1200.degree. C., of coal residues, or more precisely of slag heaps. This is ash which has been marketed under the trademark Beringites, and which is, when the product is intended for water treatment, in the form of granules of a particle size of between 0.5 and 3 mm and, when the product is intended for the treatment of solid material, in the form of a powder, described as being fine, whereas the particle size is on average the following (particle size and corresponding weight percentage):
&lt;100 .mu.m: 30.2% PA1 100-200 .mu.m: 27.5% PA1 200-500 .mu.m: 38.9% PA1 500-1000 .mu.m: 3.1% PA1 &gt;1000 .mu.m: 0.2% PA1 means for storing ash, such as a silo, and for feeding ash and means for feeding liquid or semiliquid medium to be treated, PA1 a blender adapted to being fed by the means of feeding, PA1 a centrifugal separator adapted to being fed by the blender and provided with a removal of the liquid from the treated medium and with a separate removal of the ash to which the heavy metals and the hydrocarbons which the treated medium possibly contained have, where appropriate, been bound, the ash being mixed with the solid matter which the medium contained in suspension, PA1 means for automatic control of the above-mentioned feeding, blending, centrifuging and removal operations, and PA1 an electricity generating unit suitable for supplying the plant with energy.
Beringite.RTM., which functions by adsorption and/or absorption, has been employed for the purpose of detoxification of media laden with heavy metals, with good results. However, the process for its utilization, involving a percolation, makes it necessary to employ large quantities of ash. In addition, it has not been envisaged to employ it to increase the dryness and/or the stability of sludge produced, for example, by sewage purification stations or produced by surface treatment workshops.
In addition, it has not been proposed to employ Beringite.RTM. for decontaminating liquid or semiliquid media laden with hydrocarbons, such as aqueous washes of soils soiled with polycyclic aromatic hydrocarbons, or PAHs. Until now these compounds have been removed by conventional physicochemical water treatment processes such as flocculation and flotation. The disadvantage of these known processes is that they make use of reactants which distort the results of measurement of the total hydrocarbons after treatment, results which it turns out to be necessary to obtain for a comparison with the standards in force. Furthermore, these processes do not make it possible to remove the heavy metals which the effluents being treated may also contain.